The present application relates to a nonvolatile memory device and a fabrication method thereof; and, more particularly, to a technology of resistance switching random access memory (ReRAM) using phase changes in resistance.
In general, a semiconductor memory device includes many memory cells being circuitly connected. One exemplary semiconductor memory device is a dynamic random access memory (DRAM). A typical DRAM unit memory cell consists of one switch and one capacitor, and provides benefits such as a high degree of integration and a fast operating speed.
However, since the DRAM memory cell makes two states “0” and “1” with respect to a charge, it loses all stored data when power is turned off (i.e., a volatile memory device), so it is difficult to retain data.
To facilitate data retention, research on a new memory technology attempts to make a binary state corresponding to “0” and “1” in DRAM by using a new variable, not the charge.
Nonvolatile memory devices now being studied include magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase-change ransom access memory (PRAM) and so on.
MRAM stores data using the change in magnetization direction at a tunnel junction, and FRAM stores data using polarization of ferroelectrics. Although each has merits as well as defects, both are basically known for a high integration density, a high operating speed, and an ability of low power operation, and have the potential to provide good data retention.
PRAM stores data using a change in resistance value in accordance with the phase change of a specific material, and consists of one resistor and one switch (transistor). The resistor used for PRAM is a chalcogenide resistor, which exists in a crystal state or in an amorphous state, depending on a formation temperature. Because resistance in the amorphous state is greater than that in the crystal state, a memory device may be fabricated using these characteristics. When a DRAM process is used in the fabrication of the PRAM, an etching operation may be difficult and may even take a long time. In addition, the memory device uses a transistor or diode for switching, but its structure is complicated and accurate switching operation is not easy. A simplified structure for the memory device is preferred and is being sought.
Studies and development for resistance switching random access memory (ReRAM), where high and low states of a resistance value are reproducibly switched in accordance with an applied voltage from outside, are making progress. For example, one such ReRAM device exists as an insulator in its intrinsic state, but its phase turns to a metal or semiconductor state due to an applied voltage from outside, exhibiting changes in physical properties.